In a torque converter provided with a lock-up clutch, control of the front/back differential pressure (lock-up differential pressure) of the lock-up clutch engages and disengages the lock-up clutch. When the torque converter shifts from a converter state to a lock-up state, the lock-up differential pressure may gradually increase from a predetermined initial differential pressure. Namely, the torque converter may shift from the converter state to the lock-up state via a slip state. In the converter state, the lock-up clutch is disengaged, in the slip state, the lock-up clutch slips, and in the lock-up state, the lock-up clutch is fully engaged.
Tokkai 2004-124969 published by the Japan Patent Office in 2004 discloses control of a lock-up clutch wherein lock-up is performed from a low vehicle speed and, as a result, fuel consumption is improved. This prior art relates to lock-up control in the manual mode of a continuously variable transmission. The controller which performs lock-up control has a look-up map (or look-up table) specifying, in relation to a vehicle speed VSP and throttle valve opening TVO, a region where lock-up control is performed. Furthermore, a booming noise generation region, which is located on low vehicle speed side, is specified in this map. In order that the controller can prevent generation of booming noise, slip control is performed to slip the lock-up clutch in the booming noise generation region.
Here, booming noise is a noise due to an engine torque variation being transmitted to the continuously variable transmission from the engine via the lock-up clutch, and is generated in the passenger compartment.